The acoustic-magneto (AM) technology has been used in the electronic article surveillance industry for over two decades. The early form of this AM technology is disclosed in U. S. Pat. No. 4,510,489 granted to Philip Anderson III, et al on Apr. 9, 1985, and assigned to Allied Corporation. This Anderson patent disclosed that certain amorphous ribbons demonstrated high magneto-mechanical coupling factor. Therefore, the cut ribbons can emit strong resonant signals under proper bias fields. This Anderson patent led to the commercial AM anti-theft devices that are widely used in retail markets today.
Such an AM anti-theft device includes a detecting system, markers, a marker deactivator, and marker verifiers. Currently, a widely used commercial product is marketed as the Ultra-max® system made by Sensormatic Electronics Corporation (SEC). This Ultra-max® system emits a 58 kHz pulse field to excite active markers and induce strong resonant signals around 58 kHz. The pick-up coils detect and amplify such induced resonant signals, analyzing the “ring-down” characteristics, then set off an alarm if this specific marker's signal profile is detected. The markers can be selectively deactivated to prevent the alarm from being triggered. This deactivation involves a demagnetization of the bias piece inside an activated marker, which shifts the resonant frequency away from 58 kHz and reduces the signal amplitude. Therefore, the alarm will not be triggered when deactivated markers passing through interrogation zone.
The AM anti-theft markers can be classified into two types: permanent tags and disposable labels. The permanent tag uses an amorphous ribbon as a resonator and permanent magnets (e.g. hard ferrite magnets) as the bias material that cannot be easily deactivated. Permanent tags were not as convenient to use, as the mechanical lockers have to be manually unlocked by the cashier when customers paid for the articles that were protected by attached permanent tags.
The prior disposable labels use the same amorphous ribbon as a resonator, but uses “semi-hard” magnetic materials as the bias component, which can be deactivated and re-activated repeatedly. The bias material is the key part of the AM labels and determines the frequency and amplitude of the AM labels. The bias material affects the performance of the anti-theft labels and the cost of manufacturing. Attempts have been made to develop new bias materials and to improve the manufacturing processes for the anti-theft security labels. For example, U. S. Pat. No. 4,536,229 issued to Sungho Jin, et al on Aug. 20, 1985, is directed to a cobalt free Fe—Ni—Mo semi-hard magnetic alloy suitable for security devices. In U. S. Pat. No. 5,351,033, granted to Nen-Chin Liu, et al on Sep. 27, 1994, a method for making semi-hard magnetic elements is disclosed. Magnetic strips manufactured by annealing a iron-manganese alloy and then cold-rolling the alloy and heat treating the strips to provide strips for use in EAS systems are taught in U. S. Pat. No. 5,716,460, granted to Neil Manning, et al on Feb. 10, 1998.
U.S. Pat. No. 5,729,200, issued to Richard Copeland, et al on Mar. 17, 1998, and U.S. Pat. No. 6,181,245, issued to Richard Copeland, et al on Jan. 30, 2001, taught an anti-theft marker that is formed with semi-hard bias materials with lower coercivity that can be deactivated by applying a lower level AC magnetic field. In U.S. Pat. No. 6,001,194, granted on Dec. 14, 1999, to Noriyuki Nakaoka, and in U.S. Pat. No. 6,893,511, granted on May 17, 2005, to Noriyuki Nakaoka, a method of producing a bias material for use as a magnetic marker in an anti-theft device is disclosed in which the magnetic marker is formed with a non-magnetic copper group dispersed within an iron-based matrix, thus forming a semi-hard magnetic material. U.S. Pat. No. 6,689,490, issued on Feb. 10, 2004, to Hartwin Weber, et al, discloses a method of manufacturing an activation strip for use in an anti-theft label by using a Fe—Ni—Al—Ti based semi-hard magnetic material.
In David Jile's book “Introduction to Magnetism and Magnetic Materials” published by Chapman & Hall (ISBN0-412-38640-2), page 269-270, the definition of soft magnetic material is having “a coervicity”<1000 A/m (<12.5 Oe) while hard magnetic material is having “a coercivity”>10,000 A/m (>125 Oe). The semi-hard magnetic material is a group of ferromagnetic materials that have DC coercive force (or “DC coercivity”, “DC Hc”) between that of soft magnetic materials and that of hard magnetic materials, which meanwhile have high remanence (e.g. all commercially available semi-hard magnetic materials have the Br much higher than 7 kGs). The comparable “coercivity” between various groups of magnetic materials is its intrinsic property. Coercivity measured under a DC field (DC Hc) removes non-intrinsic variable components of coercivity (e.g. frequency, sample size and shape, resistivity of the material, etc) so that the DC Hc can be used to compare the intrinsic magnetic properties of various magnetic materials. However, as we will discuss further, it is not enough to use DC Hc to judge whether a particular magnetic material is particularly suitable as bias component for an AM security label. Instead, we have to use the AC coercivity (AC Hc) that is the reverse field strength at the point on the AC B—H loops when the B=0. It is the reverse field needed to drive a particular magnetic component with special dimensions and resistivity under particular frequency and wave form. Traditionally, AC Hc is not used to measure semi-hard materials or permanent magnetic materials but usually to be used to judge soft magnetic materials.
Permanent magnets usually are brittle and are difficult to cold work; therefore, it is difficult to cold roll permanent magnetic materials into thin strips (e.g. about 0.05 mm thick). Soft magnetic metal materials are much easier to work into thin strips, but were not considered for use as the bias piece, due to concerns such as: the marker with soft magnetic material bias piece might be self-deactivated in the interrogation zone by the reversal pulse fields; or the soft magnetic bias piece might not give enough bias field strength. As a result, it has been a well-established practice that “semi-hard” magnetic materials (DC Hc>12.5 Oe) were required to be used as the bias component in AM labels. A specific example of this requirement can be found in the aforementioned U.S. Pat. No. 6,689,490, assigned to the current supplier of the semi-hard bias material used in AM labels, Vacuumschmelze GmbH, at column 1, line 44-47, “On the other hand, however, an adequate opposing field stability is also required, as a result whereof the lower limit value of the coercive force is determined. Only coercive forces of at least 10 A/cm are thereby suited”. 10 A/cm converts to 12.5 Oe. Consequently, all prior developments and related patents on materials for the bias piece used in AM labels are limited to “semi-hard magnetic materials”. No prior commercial AM labels use a bias material with its DC Hc lower than 12.5 Oe.
A “semi-hard” magnetic material generally has complicated multi-phase structure and has a ductile matrix mixed with at least one hard magnetic phase. The ductile matrix phase is needed for good cold workability; however, a low temperature (e.g. below 650° C.) aging or annealing must be used to control the hard-magnetic phase precipitation morphology and amount to get the required DC Hc and DC Br. Such low temperature heat treatments require long processing time. Furthermore, DC Hc is highly sensitive to slight temperature variations during low temperature heat treatments. In fact, to achieve high lot-to-lot consistency of the semi-hard material bias material's DC Hc and DC Br is a very challenging task in massive production. A slight temperature fluctuation within a big annealing furmace's different locations could end up with the big DC Hc and DC Br scattering in the finished strip. Most such heat treatments are non-recoverable meaning the strip with thin thickness will have to be scrapped if the final properties failed to meet the requirements. This lack of consistency could lower the yield and reduce the production reliability and, consequently, increase the cost for AM labels.
Prior AM labels (such as is described in U.S. Pat. No. 6,359,563, granted on Mar. 19, 2002, to Giselher Herzer, and shown particularly in FIGS. 3 and 4 thereof) include an elongated plastic housing and cover. The cover includes a first cover film, double side tape, a bias piece made with a semi-hard magnetic material, and another cover film. The resonating cavity inside the housing holds one or two resonator pieces. The resonator pieces will typically have a bowed shape across the width dimension. The bias piece is formed as a parallelogram with two sharp corners being cut off. All prior AM labels have used “semi-hard” magnetic materials as the bias piece. The technologies to make semi-hard magnetic material for AM labels are quite complicated, and the materials are not widely available, thus leading to the higher cost.
Furthermore, the DC Hc of the semi-hard magnetic material is higher (e.g. all semi-hard magnetic materials are higher than about 20 Oe in prior commercially available bias materials), as well as higher DC Br (e.g. generally higher than 15 kGs) than that of soft magnetic materials. The higher DC Hc and DC Br values from semi-hard magnetic bias component produce a stronger bias field that will cause unavoidable strong “clamping” effect to attract resonator pieces that reduces the resonance amplitude of the AM label. Consequently, many measures in prior AM labels have to be taken to minimize this clamping effect. Examples of such measures are: making the resonator strip with a transverse bowed shape; forming the bias piece in a non-rectangular configuration; and/or increasing the label thickness by placing the bias piece outside of the resonating cavity of the housing to create enough separating distance between the bias material and the resonator pieces.
In a high speed manufacture process, the accurate positioning of the multiple sealing plastic films for the thin semi-hard bias piece outside of the resonating cavity of the house makes the label production equipment and the manufacturing process more complex with resultant higher costs. Furthermore, the non-flat resonator strip is prone to change its resonant frequency due to slight stress or pressure changes compared to flat ones, according to Dimitris Kouzoudis et al, as is reflected in “The Frequency Response of Magnetoelastic Sensors to Stress and Atmospheric Pressure”, Smart Mater.Struct. 9(2000) pp1-5.
Prior research and development on bias materials used in AM labels were all limited to semi-hard magnetic materials. The difference between “DC coercivity” and “AC coercivity” is not appreciated. All referred “coercivity” in the prior art AM devices was only the “coercivity” being measured in direct current (DC Hc). However, it is critical to recognize that the “coercivity” measured specifically at 58 kHz alternative current (AC Hc) is a real specification to judge the stability of a particular bias thin strip with specific dimensions. AC Hc is a true material stability test for AM labels because such labels are used in a high frequency field, such as 58 kHz, instead of being used in a DC field.
DC Hc is determined by applying a reversal magnetic field slowly then measure what the peak reversal field strength is needed to fully drive the material's magnetic induction to zero. However, DC tests cannot truly reveal the behavior of the bias piece at high frequency field 58 kHz where the field strength changes very quickly. AC coercivity is the peak reversal field strength that is needed to drive a magnetic component with particular dimensions to zero magnetic induction, when a high frequency AC magnetic field is applied. DC Hc is only a material's intrinsic magnetic property while AC Hc is the combination effects of the material intrinsic magnetic properties, AC frequency and its wave form, conductivity of the material, and the size/shape of the component made with the material under test. Therefore, AC Hc is a much better measurement to be closer to reflect true performance of a particular magnetic component at a specific AC field working environment than DC Hc.
All prior works overlooked the well-known physics phenomenon in designing a bias component for an AM label, namely, the AC coercivity increases with increased frequencies, especially for a magnetic component made with lower DC Hc. The data and its mechanism can be found in many literatures, for examples: F. Marthouret et al. “Modeling of a Non-linear conductive magnetic circuit” IEEE Trans. on Mag. Vol 31, No6, pp 4065-4070 (November 1995), D. C. Jiles, “Frequency dependence of hysterisis curves in conducting magnetic materials” J. Appl. Phys. 76 (10), pp5849-5855 (November 1994), Jan Szczyglowski, “Influence of eddy currents on magnetic hysterisis loops in soft magnetic materials” J. Magnetism and Magnetic Materials, 223, pp97-102 (2001).
Accordingly, it would be desirable to provide an anti-theft security tag that can be manufactured with lower cost materials to trigger an alarm when passing through a standard detection apparatus, and is also capable of being deactivated more reliably and more easily so as to pass through the standard detection apparatus without triggering the alarm.